I’ve always regarded the manufacture of jam as messy, time-consuming work involving cauldrons of boiling syrup, clouds of sickly vapour and the ever-present threat of scalding burns. In traditional communities it is performed only by the bravest and most hardened of kitchen workers, such as grandmothers. If I were ever to get money for jam I would consider it a just compensation for my pains but the common usage of the phrase carries the mystifying implication that making jam is such a delightful and carefree employment that it is practically its own reward.
Perhaps my ideas on this subject were distorted at an early age by tales of horrific jam-related injuries.
It has been suggested that I have misinterpreted the idiom and that the jam in question is to be consumed rather than made. In other words, we are to imagine being paid to eat jam. This seems unlikely, but it is better than any of my tentative hypotheses, all of which rely on an omitted prefix of the word ‘jam’ or its homophones, and none of which (‘door’, ‘toe’, ‘The’) are particularly appealing.